I. Field of the Invention
This invention pertains to techniques for sealing concentric annular surfaces. More particularly, this invention relates to methods and apparatus for cementing liners in wells, and specifically for sealing such liners to pipe strings used to circulate fluid into the well bore during the cementing procedure.
II. Description of Prior Art
The practice of cementing tubing in well bores is well known. Sections of casing may be positioned and cemented in place from the surface as the drilling of the well progresses. Thereafter, for example, a liner may be lowered into a well suspended by a pipe string and positioned to overlap the casing or a previously cemented liner. The liner is anchored in place, usually on slips, and the setting tool by which the liner is suspended from the pipe string is released. Cement is then pumped through the pipe string to the liner, and displaced through the liner, usually through a foot valve, into the annular space between the liner and the surrounding casing or well bore. Plugs are used to separate the cement from displacing fluid, and to wipe the cement from the operating string and the liner.
In such cementing operations, it is necessary to provide a seal in the annular space between the setting tool extension of the pipe string and the surrounding liner so that fluid pressure may be maintained in the operating string during the cementing operation. Inverted swab cup type seal assemblies mounted on the setting tool mandrel for sealing engagement with the surrounding liner have been used for this purpose. The swab cups are removed with the setting tool after the liner has been cemented in place. Annular seals mounted on the interior of the liner for sealing engagement with the setting tool mandrel have also been used. Generally, such liner-mounted seals must be drilled out after the cementing operation since the seals would otherwise partially obstruct the passage through the interior of the liner. Seal assemblies have also been designed which are locked to the liner during the cementing operation, but may be released and retrieved by means of the setting tool.
Since the primary purpose of such setting tool-to-liner seal systems is to permit the establishment of sufficient pressure within the operating string to circulate the cementing fluid, such seal assemblies are particularly designed to withstand pressure differentials from below. However, if the level to which the cement is placed outside of the liner is sufficiently high relative to the seal assembly, a substantial pressure differential from above the seal assembly may develop. In such event, if the pressure differential cannot be relieved, it may be difficult to remove the seal assembly with the mandrel. It is therefore desirable to provide a means for permitting fluid pressure to bypass the seal assembly so that the seal assembly and setting tool may be removed from the well.
Additionally, during the cementing operation, if insufficient fluid is circulated into the well, an unbalanced fluid pressure may be exerted on the seal assembly from above. If the pressure differential is reduced by the opening of a bypass around the seal assembly, the sealing between the setting tool and the liner must be re-established so that the cementing operation may continue. Therefore it is desirable to provide a seal assembly permitting such fluid pressure communication with means whereby the bypass may again be closed and the liner sealed to the mandrel to withstand a pressure differential from below.